In certain industries such as electrical utilities and oil well logging industry, large amounts of data obtained from monitoring and testing are processed and analyzed to yield useful information. For example, in the electrical utility industry, data recorders monitor electrical power lines for signal fluctuations indicative of problems in the transmission line. Typical power line fault recorders record analog data from the voltages and currents on the transmission lines emanating from sub stations and gene ration stations. Some conventional data recorders operate constantly even when no problems are present, generating enormous amounts of data which yield little useful information. When a problem such as a lightning strike or a tree falling on a line or the like occurs, a skilled analyst can examine the characteristics of the data (such as rise times, transient characteristics, and durations of signals) and ascertain the location and nature of the fault. For example, when a fault is detected in an electrical power line, it is important for the data containing information about the fault to be relayed as quickly as possible to the personnel responsible for analyzing and correcting the fault. Typically, these personnel display the data graphically and look for tell-tale patterns indicative of certain common known types of faults, e.g. a tree falling on a transmission line.
Often, however, these personnel are remote from the substation or generating facility where the monitoring equipment is located. Thus, the data must be transmitted via a telephone line and modem link, or physically transported on a medium such as magnetic tape. Needless to say, transmission of large amounts of data at standard rates of 2400 baud is inordinately slow, and storage of such data on magnetic tape is not a preferable alternative because of the weight and bulk of the reels or cartridges of tape. While often the resolution of the data is not critical for graphic display, the speed of acquiring the data is essential so that the power outage time can be minimized.
In many typical power line monitoring data recorders, components known as continuous monitoring equipment ("CME") detect the occurrence of a fault condition and initiate the recording of data for subsequent analysis. Large amounts of data provided during normal operation accordingly need not be saved, resulting in some savings in transmission volume and speed.
However, the CME requires a finite amount of time to detect the occurrence of a fault. Thus, when a fault has been detected, a predetermined amount of data prior to the occurrence of the fault must somehow be saved for analysis, since the data immediately prior to the fault often provides the most useful information as to the nature of the fault. Typically, electrical utility CME continuously record data and store it in a temporary buffer so that when a fault is recognized, the data in the temporary buffer prior to the fault plus data corresponding to the fault is preserved.
Data recording for other applications such as seismic recording and nuclear testing also produce vast amounts of data which must be either recorded at high speed, buffered, or compressed for later analysis. Often, digital tape drives are employed for saving the data into a permanent storage medium for subsequent analysis. Typical magnetic tape drives used in such data recorders can operate at speeds of up to 100 inches per second, for nine tracks with a packing density of 1600 bits per inch per track. Although these are high density, high speed recorders, in some applications the amount of data being accumulated to provide the desired resolution is greater than the recording ability of the tape drives, requiring either multiple tape drives or data compression schemes.
In these and other data recording devices, data compression techniques are frequently used to reduce the volume of data prior to storage or transmission. These techniques can result in greater transmission speeds and storage densities. Many types of data compression techniques are known in the art. These techniques fall basically into two primary categories: (1) logical compression (also called "redundancy reduction") and (2) physical compression (also called "entropy reduction"). Logical compression is a data dependent technique and results from the elimination of redundant fields of information while representing data elements in remaining fields with as few logical indicators or codes as is feasible. Physical compression or entropy reduction is typically viewed as the process of reducing the quantity of data prior to it entering a transmission channel or storage medium and the subsequent expansion of such data to its original format upon receipt or recall. Physical compression necessarily results in loss of information, since data which may contain useful information is deliberately discarded.
One particular compression technique employed in seismologic recording involves a gain-switchable analog-to-digital converter which selects one of four different gain ranges for a particular data sample. Two bits of data are added to each digital data sample for encoding gain information. Thus, at the instant of sampling, a two-bit gain code is obtained representative of which one of four possible amplitude ranges is associated with the sample. The gain code is used to select one of four analog amplifiers on the input to the D/A converter prior to digitizing the sample. The digital output of the D/A converter, plus the two-bit gain code, then constitutes the reduced data. While this technique results in a degree of data compression and reduced bandwidth, it fails to take advantage of the fact that a plurality of consecutive samples may have the same gain code, resulting in the transmission of much redundant information. Thus, the compression efficiency for this scheme is not particularly great.